Marshall Brain's Blog

A friend read the previous post and sent this article on ultracapacitors as another alternative to batteries. From the article:

Maxwell's D Cell supplies bursts of high power and recharges rapidly from any energy source over hundreds of thousands of cycles without significant performance degradation. The D Cell has ultra-low internal resistance so energy is easily collected and stored for reuse. Providing reliability assurance, the ultracapacitors have a 10 year-life capability, deliver more than 500,000 discharge-recharge cycles and are resistant against reverse polarity. The D Cell has a capacitance of 350 Farads (DCC, 25 Degrees C) and is rated at 2.5 volts. The cylindrical cells are constructed of aluminum, feature a round, double-ended design measuring 61.5 x 33 mm (L x OD), similar to EN 60086-2 and EN 60285 sizing, and weigh only 60 grams.

One thing the article does not mention is the discharge rate due to leakage -- how long the capacitor can hold the charge. Certainly a capacitor will not hold a charge for weeks like a battery can, but can probably hold a charge for several hours. This makes ultracapacitors a reasonable solution for storing things like power from regenerative braking.

Note also that a typical AA alkaline battery can deliver power equivalent to a 10,000 Farad or so capacitor. There is no way a AA battery can dump its full power in a millisecond like a capacitor can, but a AA battery still represents a fairly large amount of stored electrical energy. It might take 20 of these D cell ultracapacitors to store the energy available in a single AA battery.

Compared to gasoline, things are even worse. The article boasts about an energy density of 21 joules per cc for ultracapacitors. Gasoline has an enery density of about 35,000 joules per cc, or more than 1,600 times denser. Think about how big a 20 gallon gas tank is. Multiply it in size by 1,600. That is how many ultracapacitors it would take to store the energy equivalent of a tank of gas. Since a car engine is only able to capture about 25% of the energy in the gasoline (the rest is lost as waste heat), you would only need to to store 30% of the energy of a tank of gas in ultracapaitors to have an equivalent storage capacity, but that is still a LOT of ultracapacitors.